1. Field of the Invention
The present invention relates to a light emitting device using an organic light emitting element with a film containing an organic compound that emits light with application of electric field (hereinafter referred to as organic compound layer), as well as an anode and a cathode. Specifically, the present invention relates to a light emitting device using an organic light emitting element with lower drive voltage than before and longer lifetime. The term light emitting device in this specification refers to an image display device or a light emitting device that employs as a light emitting element an organic light emitting element. Also included in the definition of the light emitting device are a module in which a connector, such as an anisotropic conductive film (FPC: flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package), is attached to an organic light emitting element, a module in which a printed wiring board is provided on the tip of a TAB tape or a TCP, and a module in which an IC (integrated circuit) is mounted directly to an organic light emitting element by the COG (chip on glass) method.
2. Description of the Related Art
An organic light emitting element is an element that emits light when electric field is applied. Light emission mechanism thereof is said to be as follows. A voltage is applied to an organic compound layer sandwiched between electrodes to cause recombination of electrons injected from the cathode and holes injected from the anode at the luminescent center in the organic compound layer and, when the resultant molecular excitons release energy in the form of light emission in returning to base state.
There are two types of molecular excitons from organic compounds; one is for a singlet exciton state and the other is for a triplet exciton state. This specification includes both cases where the singlet excitation state causes light emission and where the triplet excitation state causes light emission.
In an organic light emitting element as above, its organic compound layer is usually a thin film with a thickness of less than 1 μm. In addition, the organic light emitting element does not need back light used in conventional liquid crystal displays because it is a self-light emitting element and the organic compound layer itself emits light. The organic light emitting element therefore has a great advantage of being manufactured as a very thin and light-weight device.
When the organic compound layer is about 100 to 200 nm in thickness, for example, recombination takes place within several tens nanoseconds based on the mobility of the carriers in the organic compound layer. Even if the process from carrier recombination to light emission is taken into account, the organic light emitting element may be ready for light emission within an order of microsecond. Accordingly, fast response is also one of the features of the organic light emitting element.
Since the organic light emitting element is of carrier injection type, it can be driven with direct-current voltage and noise is hardly generated. Regarding driving voltage, a report says that a sufficient luminance of 100 cd/m2 is obtained at 5.5 V by using a very thin film with a uniform thickness of about 100 nm for the organic compound layer, choosing an electrode material which is capable of lowering a carrier injection barrier against the organic compound layer, and introducing the hetero structure (laminate structure) (Reference 1: C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes”, Applied Physics Letters, vol. 51, no. 12, 913–915 (1987)).
With those features, including being thin and light-weight, fast response, and direct-current low voltage driving, an organic light emitting element is attracting attention as a next-generation flat panel display element. In addition, for being self-light emitting device with a wide viewing angle, the organic light emitting element has better visibility and is considered as effective when used for display screens of electric appliances.
In the organic light emitting element disclosed in Reference 1, the carrier injection barrier is lowered by using a Mg:Ag alloy that is low in work function and is relatively stable as the cathode so that more electrons are injected. This makes it possible to inject a large number of carriers into the organic compound layer.
Further, a single hetero structure, in which a hole transporting layer formed of diamine compound and an electron transporting light emitting layer formed of tris(8-quinolinolate) aluminum complex (hereinafter referred to as Alq3) are layered as the organic compound layer, is adopted to improve the carrier recombination efficiency exponentially. This is explained as follows.
In the case of an organic light emitting element in which an organic compound layer consists of a single layer of Alq3, for example, most of electrons injected from a cathode reach the anode without being recombined with holes and the light emission efficiency is very low. In short, a material that can transport electrons and holes both in balanced amounts (hereinafter referred to as bipolar material) has to be used in order that a single layer organic light emitting element can emit light efficiently (i.e., in order to drive at low voltage), and Alq3 does not meet the requirement.
On the other hand, when the single hetero structure (two-layer structure) as in Reference 1 is adopted, electrons injected from the cathode are blocked at the interface between the hole transporting layer and the electron transporting light emitting layer and trapped in the electron transporting light emitting layer. Recombination of the carriers thus takes place in the electron transporting light emitting layer with high efficiency, resulting in efficient light emission.
Expanding this idea of carrier blocking function, it is possible to control the carrier recombination region. To give an example, there is a report of success in making a hole transporting layer to emit light by inserting a layer that can block holes (hole blocking layer) between the hole transporting layer and an electron transporting layer and trapping the holes in the hole transporting layer. (Reference 2: Yasunori KIJIMA, Nobutoshi ASAI and Shin-ichiro TAMURA, “A Blue Organic Light Emitting Diode”, Japanese Journal of Applied Physics, vol. 38, 5274–5277 (1999)). A hole blocking layer formed of a material as shown in Reference 2 has an excitation energy higher than that of a light emitting layer and therefore also prevents molecular excitons from diffusing.
It can be said that the organic light emitting element in Reference 1 is characterized by separation of functions in which the hole transporting layer is assigned to transport holes and the electron transporting light emitting layer is assigned to transport electrons and emit light. The idea of separating functions has been expanded until a method is proposed in which three types of functions of hole transportation, electron transportation, and light emission are conducted by three different materials. With this method, a material that scores poorly in carrier transportation but is high in light emission efficiency can be used as a light emitting material and the light emission efficiency of the organic light emitting element is accordingly improved.
The typical method thereof is pigment doping (Reference 3: C. W. Tang, S. A. VanSlyke, and C. H. Chen, “Electroluminescence of doped organic thin films”, Journal of Applied Physics, vol. 65, no. 9, 3610–3616, (1989)). As shown in FIG. 13A, in a single hetero structure provided with a hole transporting layer 1101 and an electron transporting layer 1102 (1102 also serves as a light emitting layer), the electron transporting layer 1102 is doped with a pigment 1103 to give emitted light the color of the pigment 1103. The hole transporting layer 1101 side may instead be doped with the pigment 1103.
In contrast to this, there is a double hetero structure (three-layer structure) in which a light emitting layer is sandwiched between a hole transporting layer and an electron transporting layer as shown in FIG. 13B (Reference 4: Chihaya ADACHI, Shizuo TOKITO, Tetsuo TSUTSUI and Shogo SAITO, “Electroluminescence in Organic Films with Three-layered Structure”, Japanese Journal of Applied Physics, Vol. 27, No. 2, L269–L271 (1988)). In this method, holes are injected from the hole transporting layer 1101 to a light emitting layer 1104 and electrons are injected from an electron transporting layer 1102 to the light emitting layer 1104. Therefore, recombination of the carriers takes place in the light emitting layer 1104 and light with the color of the material used as the light emitting layer 1104 is emitted.
An advantage of separating functions is an increased degree of freedom in molecule design and the like since the separation of functions saves one organic material from bearing various functions (such as light emission, carrier transportation, and injection of carriers from electrodes) simultaneously (for instance, the separation of functions makes the effort to find a bipolar material unnecessary). In other words, high light emission efficiency can easily be obtained by simply combining a material excellent in light emission characteristic with a material excellent in carrier transportation ability.
Because of these advantages, the idea itself of laminate structure described in References 1 to 4 (carrier blocking function or separation of functions) continues to be utilized widely.
However, the laminate structures as described above are joining different substances and thus cannot avoid energy barriers formed at interfaces. The energy barriers block movement of carriers at the interfaces and raise the following two problems.
One problem is that the energy barriers are pullback in further lowering drive voltage. In fact, a report says that, as for current organic light emitting element, an element with a single layer structure using a conjugate system polymer is superior in terms of drive voltage to an element with a laminate structure and hold the top data in power efficiency (unit: lm/w) (note that comparison made in the report is for light emission from singlet excitation and the report does not deal with light emission from triplet excitation) (Reference 5: Tetsuo Tsutsui, “Journal of Organic Molecular Electronics and Bioelectronics Division of The Japan Society of Applied Physics”, vol. 11, no. 1, p. 8 (2000)).
The conjugate system polymers mentioned in Reference 5 are bipolar materials and can provide the same level of carrier recombination efficiency as the materials in the laminate structures. Therefore, the drive voltage is actually lower in the single layer structure that has less interfaces than in the laminate structures if the single layer structure can provide the same level of carrier recombination efficiency by using a bipolar material or by other methods without using the laminate structure.
For example, drive voltage can be lowered by inserting a material that can lower an energy barrier at the interface with an electrode in order that more carriers can be injected (Reference 6: Takeo Wakimoto, Yoshinori Fukuda, Kenichi Nagayama, Akira Yokoi, Hitoshi Nakada, and Masami Tsuchida, “Organic EL Cells Using Alkaline Metal Compounds as Electron Injection Materials”, IEEE TRANSACTIONS ON ELECTRON DEVICES, vol. 44, no. 8, 1245–1248 (1997)). In Reference 6, drive voltage has successfully been lowered by using LiO2 for an electron injection layer.
However, issues regarding the mobility of carriers between organic materials (between a hole transporting layer and a light emitting layer, for example, and hereinafter referred to as ‘between organic layers’) have not been solved yet and are considered as the key to catch up to low drive voltage of the single layer structure.
The other problem caused by the energy barriers is an influence on the element lifetime of the organic light emitting element. In other words, the luminance is lowered by inhibited carrier injection and the resultant accumulation of charges.
Although there is no theory that explains the mechanism of this degradation clearly, a report says that lowering of luminance can be limited by inserting a hole injection layer between an anode and a hole transporting layer and by ac driving at square wave instead of dc driving (Reference 7: S. A. VanSylke, C. H. Chen, and C. W. Tang, “Organic electroluminescent devices with improved stability”, Applied Physics Letters, Vol. 69, No. 15, 2160–2162 (1996)). This is verification by experiments, that lowering of luminance can be limited by avoiding accumulation of charges through insertion of a hole injection layer and ac driving.
Concluded from the above is that the laminate structures can readily enhance the carrier recombination efficiency and can widen the choice of materials from the standpoint of separation of functions, however, on the other hand, hinder movement of carriers and influence drive voltage and lowering of luminance because there are many interfaces between many organic layers.